The "sweet" side of a long pentraxin: how glycosylation affects PTX3 functions in innate immunity and inflammation.

Department of Immunology and Inflammation, Humanitas Clinical and Research Center Rozzano, Italy.

Abstract

Innate immunity represents the first line of defense against pathogens and plays key roles in activation and orientation of the adaptive immune response. The innate immune system comprises both a cellular and a humoral arm. Components of the humoral arm include soluble pattern recognition molecules (PRMs) that recognize pathogen-associated molecular patterns and initiate the immune response in coordination with the cellular arm, therefore acting as functional ancestors of antibodies. The long pentraxin PTX3 is a prototypic soluble PRM that is produced at sites of infection and inflammation by both somatic and immune cells. Gene targeting of this evolutionarily conserved protein has revealed a non-redundant role in resistance to selected pathogens. Moreover, PTX3 exerts important functions at the crossroad between innate immunity, inflammation, and female fertility. The human PTX3 protein contains a single N-glycosylation site that is fully occupied by complex type oligosaccharides, mainly fucosylated and sialylated biantennary glycans. Glycosylation has been implicated in a number of PTX3 activities, including neutralization of influenza viruses, modulation of the complement system, and attenuation of leukocyte recruitment. Therefore, this post translational modification might act as a fine tuner of PTX3 functions in native immunity and inflammation. Here we review the studies on PTX3, with emphasis on the glycan-dependent mechanisms underlying pathogen recognition and crosstalk with other components of the innate immune system.

Model of the PTX3 protein and its glycosylation. (A) Schematic representation of the PTX3 protomer subunit showing the N-terminal domain in yellow, followed by the globular pentraxin domain in red. Positions of the Cys residues, the N-glycosylation site at Asn220 and the pentraxin signature motif are indicated. Arrows point to a topological drawing of the N-terminal domain, which is believed to be composed of a globular region and three α-helical segments (α1, α2, and α3), and a three-dimensional model of the C-terminal pentraxin domain based on the crystal structure of C-reactive protein (Protein Data Bank ID: 1b09). (B) Disulfide bond organization of the PTX3 octamer. Highlighted are the Cys residues that participate in disulfide bond formation. The α-helical regions of the N-terminal domains are predicted to form coiled-coil-like structures, which are hypothesized to adopt either an extended (bottom) or a compact (top) conformation. (C) Different views of the SAXS scattering envelope and a schematic model of PTX3 based on the two different structural arrangements proposed for the N-terminal domain. The α-helical segments of the N-terminal domain are depicted as yellow rods. The C-terminal pentraxin domains are in red. (D) Molecular dynamics simulations indicate that the PTX3 oligosaccharides, here represented by a core monofucosylated and desialylated biantennary glycan, can adopt different conformations (orange, green, and purple), where terminal residues of sialic acid can contact specific amino acids (ball-and-stick) at the protein surface (see text for details).

Glycosylation as a tuner of PTX3 functions in innate immunity. A number of both somatic and immune cell types produce PTX3 at sites of infection/inflammation. The glycosylation status of PTX3 (e.g., branching and sialylation) might change depending on cellular source and inducing stimuli (a). In addition, the protein oligosaccharides might undergo processing by glycosidases, including neuraminidase, which are expressed or mobilized on the surface of both pathogens and host cells (e.g., neutrophils) (b). Desialylated PTX3 has higher affinity for C1q but loses recognition of ficolin-1 and influenza virus (c).